The flexibility of any microcomputer system is determined by the number and variety of the peripheral devices that can be connected to and used by the processor of the host computer system.
Desk top microcomputers consist essentially of a host computer system having a microprocessor, a memory and a direct memory access controller, interconnected by a system of busses comprising an address bus, a data bus and a control bus, together with interfaces to connect the host computer system to peripheral devices.
The peripheral devices of such systems can be used to provide input data (e.g. a scanner) for the microprocessor or to accept output data (e.g. a printer) from the microprocessor. Peripheral devices are also used for file storage which the microprocessor can both read from and write to.
In order to connect a peripheral device to the microprocessor bus system a peripheral interface is required for each peripheral device. Externally each interface takes the form of an electrical connector socket which brings the electronic signals, at the appropriate voltage levels, to the specified pin positions. Internally a peripheral interface is required to provide an intercommunication mechanism allowing the microprocessor to operate and communicate with the peripheral device using control and data signals on the microprocessor bus. The peripheral interface may be a dedicated interface arrangement, specific to a peripheral device, or a general purpose interface, usable with one of a number of different peripheral devices. There are two main types of general purpose interfaces: parallel and serial.
The parallel interface comprises a number of data ports (typically eight or sixteen) permitting the transfer of eight or sixteen bit words simultaneously, together with data flow control and status signal paths. This type of parallel interface is used for keyboard and printer interfaces, for example, and can be adapted for use with mass storage devices, robot arms, motors, valves or lamp-displays. One such interface, originally developed for printers, is referred to as the Centronics interface.
The serial interface uses a single wire for transmitting data one bit at a time to a receiver. Serial interfaces normally conform to internationally agreed standards (such as RS-232C or RS-449) which among other things, specify connector size, allocation of pins, voltage levels and communication protocols (e.g. rates of transmitting bits). RS-232C interfaces usually provide both a transmitter and a receiver circuit which are capable of independent and simultaneous operation.
Clearly, the parallel and serial general purpose interfaces provide a restriction upon the efficiency of the operation of a particular peripheral device which would not be applied if a dedicated interface was provided for each peripheral device. With a general purpose interface, the specified interface protocols have to be conformed to regardless of the most efficient protocol for the specific peripheral device. However, it becomes physically difficult and expensive to provide discrete dedicated interfaces for the wide range of peripheral devices which may be used with a desk top microprocessor system. Indeed most personal computers are only provided with a limited number, two with expansion up to six, of external peripheral interface connection points.
It is an object of the present invention to overcome the limitation on the provision of discrete interface devices in microcomputer systems and to enable more efficient use of an interface arrangement for such systems